A Simple Method to Reduce Torque Ripple in Direct Torque Controlled Permanent Magnet Synchronous Motor by Using Voltage Vectors with Variable Amplitude and Position

In this paper, a modified direct torque control (DTC) for permanent magnet synchronous machine is proposed, which enables important torque and flux ripple reduction by using voltage vectors with variable amplitude and angle. Conventional DTC presents some drawbacks, including large torque ripple, variable switching frequency and acoustic noise. The reason lies in that t h e switching table is composed of a limited number of discrete voltage vectors with fixed amplitude and position. Moreover, the selected vector will work during the whole sampling period, a n d hence their effects on torque and flux may usually be too large or too small. In the proposed DTC, the amplitudes of torque and flux errors are differentiated and they are employed to regulate the amplitude and position of the output voltage vectors on-line, which would finally be synthesized by space vector modulation (SVM). Two simple formulas are developed to derive the amplitude and position of the commanding voltage vectors from the errors of torque and flux. Conventional switching table and hysteresis controllers are eliminated and fixed switching frequency is obtained with the help of SVM. Stator flux is estimated from an improved voltage model, which is based on a low-pass filter with compensations of the amplitude and phase. The proposed DTC exhibits excellent dynamic performance and significant torque ripple reduction at steady state, which are validated by the presented simulation and experimental results.